Functional reactive programming types: Event(-Stream) / Behavior

frp.js

// data constructor

const Data = name => Dcons => {
  const Data = k => {
    const t = new Tcons();
    t[`run${name}`] = k;
    t.tag = name;
    return t;
  };

  const Tcons = Function(`return function ${name}() {}`) ();
  return Dcons(Data);
};

// reactive types

const Event = Data("Event") (Event => k => Event(k));

const Behavior = Data("Behavior") (Behavior => k => Behavior(k));

// helper

const subscribe = o => {
  o.target.addEventListener(
    o.type,
    o.listener,
    o.options
  );

  return () => o.target.removeEventListener(
    o.type,
    o.listener,
    o.options
  );
};

// Behavior instance

const ButtonPressed = initialState => {
  let state = initialState;

  const cancelDown = subscribe({
    target: document,
    type: "mousedown",
    listener: _ => state = true,
    options: {capture: true}
  });

  const cancelUp = subscribe({
    target: document,
    type: "mouseup",
    listener: _ => state = false,
    options: {capture: true}
  });

  return Object.assign(
    Behavior((k, e) => k(state)),
    {cancel: () => (cancelDown(), cancelUp())}
  );
};

// Event instance

const mouseCoords = Event((k, e) => subscribe({
  target: document,
  type: "mousemove",
  listener: event => buttonPressed.runBehavior(b => b ? k({x: event.screenX, y: event.screenY}) : null),
  options: {capture: true}
}));

// Event combinators

const comp = f => g => x => f(g(x));

const map = f => tk =>
  Event((k, e) => tk.runEvent(x => k(f(x)), e));

const filter = p => tk =>
  Event((k, e) => tk.runEvent(x => p(x) ? k(x) : null, e));

// build a lazy evaluated action
// stream even mouse coords
// in the format "#x#"
// but only if the right button is pressed

const action = comp(map(coords =>
  `${coords.x}x${coords.y}`))
    (filter(coords =>
      (coords.x & 1) === 0 && (coords.y & 1) === 0))
        (mouseCoords);

// register the Behavior with false as initial value

const buttonPressed = ButtonPressed(false);

// register the Event Stream

const cancel = action.runEvent(console.log);

console.log("1. Copy to the browser console (chromium tested only)");
console.log("2. Position cursor within document");
console.log("3. Press a mouse button and move the cursor...");
console.log("4. Is canceled after 10 secs");

setTimeout(() => (cancel(), buttonPressed.cancel()), 10000);